Support for continued investigation of the role of the nucleolus in ribosome assembly in HeLa cells is requested. Precise measurements of the lifetime of each major class of ribosomal RNA precursors will be made by methods that have already been applied to other types of RNA in this laboratory. Similar measurements will be made of the lifetime of each ribosomal protein in the nucleolar ribosomal precursor particles (80S and 55S). Nucleolar pools of each ribosomal protein will be quantitated. The results should allow us to predict at least the major aspects of the sequence of assembly of the precursor particles, i.e., proteins that associate with the RNA during transcription should be kinetically distinguishable from those that associate with the RNA later. Proteins that may function as limiting factors in the assembly of ribosomes, i.e., those with negligible pools, will be identified.